Modulation systems that move a liquid by means of an external electric field have been studied for use as image display devices and optical devices such as optical shutters, optical pickup devices, and liquid optical lenses. Typical modulation systems include electroosmosis systems, electrophoretic systems, electrofluidic systems, and electrowetting systems.
Among them, electrofluidic systems and electrowetting systems have been studied for use as low-power-consumption image display devices because they have high contrast ratios and wide viewing angles and require no frontlight or backlight. The mechanism, as discussed in PTLs 1 and 2, is as follows: based on the concept termed “electrocapillarity”, a droplet of a colored liquid present in an uncolored liquid is spread or contracted (or a droplet of an uncolored liquid present in a colored liquid is spread or contracted) by applying or not applying a voltage, thereby forming a colored image.
Such a multi-phase liquid containing a colored liquid and an uncolored liquid (hereinafter referred to as “multi-phase colored liquid”) generally contains a nonpolar solvent such as silicone oil and a polar solvent such as water, alcohol, or ethylene glycol because the two liquids need to be separated, i.e., immiscible, and a colorant is added to one of the solvents.
As an example of the addition of a colorant to a polar solvent, PTL 3 discloses the use of a colored liquid containing a polar solvent to which are added an ionic liquid containing a room-temperature molten salt composed of a combination of a cation and an anion and a self-dispersible pigment having a functional group such as a carboxyl, hydroxyl, carbonyl, sulfone, hydroxyl, or phosphate group. PTL 4 discloses the use of a colored liquid that contains a polar solvent having a particular viscosity and surface tension and having a pigment or dye added thereto and that has a particular electric conductivity and ionic radius.
As an example of the addition of a colorant to a nonpolar solvent, PTL 5 discloses the use of a colored liquid containing a nonpolar solvent such as decane, decalin, or tetralin to which are added an organic pigment and/or inorganic pigment, a solvent-soluble or solvent-dispersible polymer dispersant, and an aldehyde resin or ketone resin.